Frozen heating, ventilation, and air conditioning (HVAC) water coils can occur during cold seasons and this may cause problems in liquid phase heat transfer systems. There are two general types of heat transfer systems—liquid phase and vapor/liquid phase. Liquid phase systems operate by transfer of sensible heat or a change in temperature. Vapor/liquid systems transfer heat with the heat of vaporization by a boiling condensing cycle.
As water expands into ice, it increases in volume by about 9% and consequently may create excessive force on any pipe or valve containing it. Heating and cooling coils typically consist of serpentined rows of tubes (usually copper) that pass through sheets of formed fins (usually aluminum). As air passes through the coil and contacts the fin surfaces, heat transfers from the air to the water in the tubes. Since the heating and cooling coils consist of a significant length of tubes that can have contact with potentially very cold air, the water in these coils is susceptible to freezing.
In addition to any damage to the water coils, lines, and fittings extensive building damage can occur to the walls, ceiling, and building contents due to the freezing of water in the coils. When this occurs, often the water will leak out until it is detected and the water supply shut off. The water supply shut off also disrupts other portions of the building because the centralized piping system may serve sections of the building with multiple rooms such as complete floors. The leakage also may not be noticed immediately because it may have occurred when the building was not occupied, or may be in a location that is not readily accessible, allowing for the possibility of extensive water damage before the leak is detected.
With conventional HVAC systems, commonly-used water coil freeze protection methods have been to combine several components together to obtain freeze protection based on the correlation between a local air temperature and the water temperature in the coil and connected pipes, and then utilize a full uncontrolled flow rate when a potential freeze condition is sensed. Conventional freeze protection methods are typically used for protection of domestic water pipes from a freeze condition, and these systems are not necessarily ideal for use in HVAC water coil systems. Some conventional freeze protection systems are limited in that they estimate the water temperature from a nearby air temperature, and typically require multiple devices to be mounted and connected, thereby increasing the installation time, system cost, and the number of devices to maintain.
Furthermore, with conventional HVAC systems, the control valves may have a maximum flow setting that can be locally set, but which is not adjustable from a remote location. Further, many conventional HVAC systems cannot properly handle HVAC control applications involving top pipe changeover heating/cooling water systems, seasonal changeovers, and energy synchronization with a building's centrifugal chillers and condensing boilers, or with the building management system (BMS). A building management system implemented using a communications network is disclosed in U.S. Patent Publication No. 2010/0142535, the teachings and disclosure of which is incorporated herein by reference thereto.
Embodiments of the present invention represent an advancement over the state of the art with respect to HVAC systems and the control thereof. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.